Why was becquerel’s experiment a first step in discovering radioactivity?

Henri Becquerel in 1896 used naturally fluorescent minerals to examine the characteristics of X-rays. He used potassium uranyl sulfate and placed it on the photographic plates covered in black paper, considering that the uranium captivated energy from the Sun and then emitted it as X-rays.  

However, the hypothesis was disapproved due to overcast conditions in Paris. Due to certain reason, Becquerel decided to create his photographic plates, and it was witnessed that the images were clear and robust, claiming that the uranium discharged radiation in the absence of an external source of energy like Sun. Thus, radioactivity got discovered.  

Therefore, Becquerel experiment is considered as the initial step in discovering radioactivity because it demonstrated that a substance could continuously discharge radiation even if it was not exposed to light.